1. Field of the Invention
This invention relates to an antiskid pressure reducing device which lowers braking pressure to prevent wheels of a vehicle from coming into a locked state during brake application to the vehicle.
2. Description of the Prior Art
Generally, an antiskid pressure reducing device of the above stated type is provided with a power piston of a differential pressure motor, the power piston being arranged to move back and forth according as differential pressure between a high pressure chamber and a low pressure chamber increases or decreases; a plunger which is arranged to have one end thereof pressed to engage with the power piston by a hydraulic action of a pressure reducing chamber inserted in a braking hydraulic pressure transmission line, the plunger thus being arranged to follow the forward and backward movement of the power piston; and a valve mechanism which is arranged to open and close the braking hydraulic pressure transmission line according as the plunger moves following the power piston. When an antiskid control circuit detects a skidding state of a wheel during a braking operation on a vehicle, the differential pressure between the high and low pressure chambers of the differential pressure motor is increased to have the power piston move forward and, since there has thus taken place a hydraulic action in the pressure reducing chamber then, the plunger follows the forward movement of the power piston to cause the valve mechanism to shut off the braking hydraulic flow line. Then, the volume of the pressure reducing chamber is increased to lower the braking hydraulic pressure. Following that, when the wheel is released from the locked state, the differential pressure between the higher and lower pressure chambers of the differential pressure motor is lowered to allow the braking hydraulic pressure again to increase by an action opposite to the action performed for reduction in the braking hydraulic pressure; and then the valve mechanism is caused again to open the flow line.
In such arrangement, in order to ensure a satisfactory operation, it is necessary to keep the plunger slidable following the power piston without fail. The antiskid pressure reducing device of this type is generally arranged to operate when a vehicle is on a road surface having low frictional resistance. In reality, the use of the device is seldom required with the exception of a winter season during which road surfaces are frozen. Accordingly, the plunger which is made of metal is usually left unused over a long period of time and thus tends to rust and stick to the cylinder with which it is arranged to slidably engage. The probability of such sticking of the plunger has been presenting a threat of malfunction of the antiskid device. Further, the conventional antiskid pressure reducing devices have been provided with operation check mechanisms. However, most of these check mechanisms have been arranged to check up the operation of a power cylinder. They are not arranged to perform a check-up action under a condition in which braking hydraulic pressure is actually applied and, in effect, have not been performing any check-up for the sliding movability of the plunger.